1. Field of the Invention
The present invention generally relates to an image processing method, an image processing apparatus and a recording medium, and in particular, to an image processing method, an image processing apparatus and a recording medium employing an error diffusion method for quantizing multi-tone image data.
2. Description of the Related Art
In many cases, a dither method or an error diffusion method is employed in quantization of multi-tone image data for falsely reproducing tones of a multi-tone image in a laser printer, a digital copier, a display device, and other various image processing apparatus, for example.
Generally, the dithering method has advantages such that graininess is superior and it is possible to represent a halftone image smoothly. However, by such an area halftoning method, typically, the dithering method, resolution is degraded for achieving halftoning. Further, by the ditherintg method by which a cyclic image is generated, a moiré pattern is likely to be generated when a printed image such as that consisting of halftone spots is processed.
On the other hand, when the error diffusion method is used, it is possible to obtain resolution close to an original image, and the method is suitable for reproducing a character/letter image.
However, in the error diffusion method, when a halftone image such as a photograph is processed, isolated dots are dispersed, or dots are arranged continuously irregularly, and, thereby, graininess is not satisfactory. Further, peculiar textures may be generated, as is well known.
Further, in an electrophotographic printer, a printed image is likely to be not stable because the image is formed of isolated dots. In particular, when the error diffusion method is used in such a printer, a rate of occurrences of small isolated dots increases, thereby, the instability further increases, and degradation of graininess due to shade unevenness and banding (band-like shade unevenness) are likely to occur. With regard to the error diffusion method, in order to eliminate generation of textures due to irregular continuation of dots, the following modification arts, such as that in which a dither threshold is used as a quantization threshold so as to disturb continuation of dots, for example, have been proposed:
(1) An amount of error diffusion is increased as an edge amount increases for the purpose of eliminating generation of pseudo-contours and special stripe patterns (Japanese Laid-Open Patent Application No. 3-34772);
(2) For the purpose of preventing generation of a white blank at a non-edge, light-shade portion and preventing generation of notches in characters, a fixed threshold is used for an edge portion of an image, while a variable threshold is used for a non-edge portion, and the level of the variable threshold is lower as the shade is lighter (Japanese Patent No. 2755307); and
(3) For the purpose of preventing generation of moiré patterns and pseudo-contours when a multi-level printer of more than two levels is used, a dither signal having a magnitude according to an edge amount is added to image data for an edge portion of an image, while a fixed value is added to image data for a non-edge portion, and, the image data thus obtained is quantized into multilevel digital data using a fixed threshold (Japanese Patent No. 2801195).